When a piezoelectric single-crystal device is, for example, a rectangular plate having an aspect ratio (a/b) of 2.5 or more (a/b≧2.5, a>>L, b>>L) as shown in FIG. 1 and a voltage is applied to the polarization direction 3, an electromechanical coupling factor k31 in the lateral vibration mode is proportional to a square root of a conversion efficiency of an electric energy and a mechanical energy relating to the magnitude of vibration (lateral vibration) in the direction 1 orthogonal to the polarization direction 3. The greater k31 value is, the higher the conversion efficiency is. Examples of the shape of the piezoelectric single-crystal device include the above-mentioned rectangular plate, a square plate, a circular plate, and a bar. The electromechanical coupling factor k31 can be similarly determined for each differently shaped device.
Conventionally, as a material constituting the piezoelectric device, lead zirconium titanate (Pb(Zr, Ti)O3: PZT) has been widely used as described in T. Ogawa, M. Matsushita, Y. Tachi, and K. Echizenya, “Program Summary and Extended Abstracts of the 10th US-Japan Seminar on Dielectric and Piezoelectric Ceramics” (Sep. 26-29, (2001), pp. 245-248). However, in the device using lead zirconium titanate (PZT) disclosed in the paper of Ogawa, et al., the electromechanical coupling factor k31 is about 30%.
With a view to achieving an electromechanical coupling factor k31 value higher than that of PZT, a piezoelectric porcelain composition is disclosed in Japanese Unexamined Patent Application Publication No. 11-171644, for example. The piezoelectric porcelain composition includes x(Pb2Me2O7)½.(1-x)[Pb(Zr1-yTiy)O3] as a primary component and Cr and Si as accessory components. However, the electromechanical coupling factor k31 of the piezoelectric porcelain composition disclosed in Japanese Unexamined Patent Application Publication No. 11-171644 is 40% or less.
In addition, Jpn. J. Appl. Phys. 90 (2001), pp. 3471-3475 discloses piezoelectric properties of a single crystal, 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3, by measuring the lateral vibration mode k31 in the [100] direction or the [010] direction when the polarization direction is the [001] direction. The electromechanical coupling factor k31 is 59%.